Constructor
- Definition
- A constructor in VB .NET is a special method named New that is automatically executed when an object of a class is created and is used to initialize the data members of the class.
- Characteristics of a Constructor in VB .NET
- It is mainly used to initialize data members of a class.
- In VB .NET, the constructor is defined using the keyword Sub New().
- The constructor name is always New.
- It is executed automatically when an object of a class is created.
- It is used to initialize variables.
- It does not return any value.
- If no constructor is defined, VB .NET automatically provides a default constructor.
- It can be overloaded.
- It can have access modifiers (Public, Private, Protected).
- Syntax
Class ClassName
Public Sub New()
‘ Constructor codes
End Sub
End Class
Destructor
- Definition
- A destructor in VB .NET is a special method named Finalize() that is automatically invoked by the garbage collector to release unmanaged resources held by an object.
- Characteristics of Destructor
- In VB .NET, a destructor is implemented using the Finalize() method. Here, Name is always Finalize.
- VB .NET does not support C++-style destructors.
- Finalize() is rarely used now. In place of this method, the Dispose() method is preferred for resource management.
- It is called automatically by the Garbage Collector.
- It is used to free unmanaged resources (files, database connections, etc.)
- It cannot have parameters.
- It cannot be overloaded.
- It cannot be called directly by the programmer.
- Only one destructor per class is allowed.
-
- Syntax
Protected Overrides Sub Finalize()
‘ Cleanup code
MyBase.Finalize()
End Sub
- Use of Destructor
- Destructor is used
- To close file handles
- To release database connections
- To free unmanaged memory
- To clean up system resources
- Destructor is used
NB : Destructor call depends on the Garbage Collector(GC), hence output may vary because garbage collection is automatic.
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Program Examples
Example : A Constructor example in VB.NET to demonstrate the default and parameterized constructor concept.
Imports System
Module Module1
Public Class Person
Dim name As String
Dim salary As Integer
Public Sub New()
name = "Robert"
salary = 75000
Console.WriteLine(name)
Console.WriteLine(salary)
End Sub
End Class
Sub Main()
Dim obj1 As New Person()
Console.ReadKey()
End Sub
End Module
Output:
Robert
75000
--------------- OR -----------------
Imports System
Module Module1
Public Class Person
Dim name As String
Dim salary As Integer
Public Sub New()
name = "Robert"
salary = 75000
End sub
Public sub display()
Console.WriteLine(name)
Console.WriteLine(salary)
End Sub
End Class
Sub Main()
Dim obj1 As New Person()
obj1.display()
Console.ReadKey()
End Sub
End Module
Output:
Robert
75000
--------------- OR -----------------
'Parameterized Constructor
Imports System
Module Module1
Public Class Person
Private name As String
Private age As Integer
Public Sub New(ByVal name1 As String, ByVal age1 As Integer)
name = name1
age = age1
End Sub
Public Sub display()
Console.WriteLine(name)
Console.WriteLine(age)
End Sub
End Class
Sub Main()
Dim obj As New Person("John", 60)
obj.display()
Console.ReadKey()
End Sub
End Module
Output:
John
60
--------------------- OR --------------------
Imports System
Public Class Message
Public Str As String
Public Sub New(ByVal X As String)
Str = X
End Sub
Public Function display() As String
Return Str
End Function
End Class
Module Module1
Sub Main()
Dim con As New Message("Hello world") 'con object creation'
Console.WriteLine(con.display()) 'display method calling'
Console.ReadKey()
End Sub
End Module
-------------------------- OR -----------------------------
Imports System
Module Module1
Sub Main()
Dim con As New Message("Hello world") 'con object creation'
Console.WriteLine(con.display()) 'display method calling'
Console.ReadKey()
End Sub
End Module
Public Class Message
Public Str As String
Public Sub New(ByVal X As String)
Str = X
End Sub
Public Function display() As String
Return Str
End Function
End Class
-------------- OR ---------------
Imports System
Public Class Message
Public x, y As Integer
Public Sub New()
Console.WriteLine("Function Return type Constructor")
End Sub
Public Sub New(ByVal m As Integer, ByVal n As Integer)
x = m
y = n
End Sub
Public Function display() As String
Return x
End Function
Public Function output() As String
Return y
End Function
Public Function final() As String
Console.WriteLine(display())
Console.WriteLine(output())
Return 0
End Function
End Class
Module Module1
Sub Main()
Dim obj1 As New Message() 'obj1 object creation'
Dim obj2 As New Message(20, 30)
obj2.final()
Console.ReadKey()
End Sub
End Module
Output:
Function Return type Constructor
20
30Example: A Constructor Example in VB.NET to demonstrate the concept of constructor overloading.
Imports System
Module Module1
Public Class General
Private x, y, z As Integer
'Dim x, y, z As Integer
Public Sub New()
Console.WriteLine("Constructor overloading")
End Sub
Public Sub New(ByVal x1 As Integer)
x = x1
Console.WriteLine()
Console.WriteLine(x)
End Sub
Public Sub New(ByVal x2 As Integer, ByVal y1 As Integer)
x = x2
y = y1
Console.WriteLine()
Console.WriteLine(x)
Console.WriteLine(y)
End Sub
End Class
Sub main()
Dim obj1 As New General()
Dim obj2 As New General(100)
Dim obj3 As New General(500, 1000)
Console.ReadKey()
End Sub
End Module
Output:
Constructor overloading
100
500
1000Imports System
Class A
Shared Sub New()
Console.WriteLine("Class A constructor initiated")
End Sub
End Class
Class B
Inherits A
Shared Sub New()
Console.WriteLine("Class B constructor initiated")
End Sub
Public Shared Sub Message()
Console.WriteLine("Hello world")
End Sub
End Class
Module Testcons
Sub Main()
Console.WriteLine("50")
B.Message()
Console.WriteLine("100")
Console.ReadKey()
End Sub
End Module
Output:
50
Class B constructor initiated
Hello world
100Example : A VB .Net program to demonstrate the Constructor and Destructor concept.
Class Sample
'Destructor only
Protected Overrides Sub Finalize()
Console.WriteLine("Destructor called")
MyBase.Finalize()
End Sub
End Class
Module Module1
Sub Main()
Dim obj As New Sample()
obj = Nothing
GC.Collect()
GC.WaitForPendingFinalizers()
End Sub
End Module
----------------- OR -----------------
'Constructor and Destructor Program
Class Example
' Constructor
Public Sub New()
Console.WriteLine("Constructor is called")
End Sub
' Destructor
Protected Overrides Sub Finalize()
Console.WriteLine("Destructor is called")
MyBase.Finalize()
End Sub
End Class
Module Module1
Sub Main()
Dim obj As New Example() 'Constructor executed due to Object creation
obj = Nothing 'Object becomes eligible for destruction
GC.Collect()
GC.WaitForPendingFinalizers()
Console.ReadLine()
End Sub
End Module
----------------- OR -----------------
Class Example
Implements IDisposable 'Tells .NET that this class supports resource cleanup.
Public Sub New()
Console.WriteLine("Object Created")
End Sub
' Dispose method release unmanaged resources manually
Public Sub Dispose() Implements IDisposable.Dispose
Console.WriteLine("Resources Released by Dispose() method calling")
End Sub
End Class
Module Module1
Sub Main()
Dim obj As New Example()
' Explicitly releasing resources
obj.Dispose()
Console.ReadLine()
End Sub
End Module
Output:
Object Created
Resources Released by Dispose() method calling
NB: Here Dispose() called explicitly by programmer, not by Garbage Collector(GC).
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